Olefins are readily available and inexpensive raw materials for industrial applications. They can be converted by oxidation into epoxides which are in turn of great importance as versatile intermediates in the synthesis of active compounds and fine chemicals (cosmetics industry, polymer industry, etc). Chiral epoxides are particularly valuable since they make it possible to obtain numerous structural elements, e.g. chiral diols or chiral amino alcohols, which frequently occur, in particular, in natural products and biologically active compounds. The most efficient and most economical method of synthesizing chiral epoxides is the catalytic, asymmetric epoxidation of olefins.
Mezetti et al. (Green Chemistry 1999, 1, 39-41; Organometallics 2000, 19, 4117-4126) discloses that olefins can be epoxidized in the presence of ruthenium catalysts and using hydrogen peroxide as oxidant, but the enantioselectivities of at most 42% are unsatisfactory and the yields are in most cases too low for industrial applications.
Other ruthenium-based systems, e.g. chiral Schiff-base complexes (Kureshi et al., J. Mol. Catal. A: Chemical 1997, 124, 91-97) are likewise quite inefficient in terms of yield and stereoselectivity.
A further process has been described by Nishiyama et al. (Chem. Commun. 1997, 1863-1864). In the epoxidation of trans-stilbene, enantioselectivities of 74% at yields of up to 63% were achieved using ruthenium-pyridine-2,6-dicarboxylate complexes having chiral bis(oxazolinyl)pyridine ligands. However, a disadvantage of this process is that bis(acetoxy)iodobenzene has to be used as reoxidant. This reagent is unattractive for industrial processes because of its high price.
There is therefore still a need to develop a general and efficient process for the asymmetric epoxidation of olefins, which displays high chemoselectivities and enantioselectivities and also gives good product yields. At the same time, the use of a both inexpensive and environmentally friendly oxidant is desirable from an industrial point of view.